The phenomenon of photochromism has been known for many years. A compound is said to be photochromic when, after irradiation with a light beam certain wavelengths of which are situated in the ultraviolet range, this compound changes colour and returns to its original colour once the irradiation is stopped.
This phenomenon has many applications, but one of the more particularly advantageous known applications relates to the field of ophthalmic optics.
Such compounds may be used in the production of lenses or glass for spectacles in order to filter light radiations depending on their intensity.
By incorporating photochromic compounds into an organic material constituting an ophthalmic lens, it is possible to obtain a glass of considerably reduced weight relative to conventional lenses made of inorganic glass which contain silver halides as photochromic agent. Their incorporation into organic materials has always posed technical difficulties.
However, not all compounds with photochromic properties are necessarily usable in the field of ophthalmic optics. Indeed, the photochromic compound must fulfil a certain number of criteria, some of which are:
strong colorability, which is a measurement of the capacity of a photochromic compound to show an intense colour after isomerization; PA1 a coloration after absorption of light, which renders the photochromic compound capable of being used, alone or in combination with other photochromic compounds, in ophthalmic glass or lenses; PA1 an absence of coloration, or very little coloration, in the initial form; PA1 rapid coloration or decoloration kinetics; PA1 a photochromism exhibited in the widest possible temperature range, and in particular preferably between 0.degree. and 40.degree. C.
The organic photochromic compounds currently known and used generally show decreasing photochromism as the temperature rises, such that the photochromism is particularly pronounced at temperatures close to 0.degree. C., whereas it is much weaker, or even non-existent, at temperatures of the order of 40.degree. C., which are temperatures which the glass may reach especially on exposure to the sun.
Another problem encountered by the photochromic compounds of the state of the art is their lifetime. Indeed, for certain products of the state of the art, a relatively short lifetime is observed. In effect, after a certain number of coloration and decoloration cycles, the photochromic compound becomes blocked generally in an open and coloured form and no longer displays reversible photochromic properties.
The photochromic properties of spirooxazines have been described by R. E. Fox in the document Final Report of Contact AF 41, A.D. 440226 1961, 657.
Compounds of spiro(indoline-naphthooxazine) type have been synthesized and described in particular in the article by N. Y. C. Chu "Photochromism: Molecules and Systems" Ed. H. Durr, H. Bovas Laurent, Elsevier, Amsterdam 1990, ch. 24, and compounds of spiro(indoline-quinazolinooxazine) or spiro(indoline-benzothiazolooxazine) type in U.S. Pat. Nos. 5,139,707 and 5,114,621 (R. Guglielmetti, P. Tardieu) filed in the name of the company ESSILOR.